This study is designed to test the hypothesis that temozolomide can be safely administered to pediatric and adolescent patients with refractory cancer as a daily oral dose for 5 days. The following specific aims will be addressed in this Phase I study to test this hypothesis: 1)determination of the maximum tolerable dose of temozolomide given orally daily for 5 days; 2) determination of the toxicities of temozolomide given on this schedule; 3) determination of the pharmacokinetic behavior of temozolomide in pediatric and adolescent patients; and 4) determination of the antitumor activity of temozolomide within the confines of a Phase 1 study.